This disclosure relates generally to surgical devices and procedures, and more particularly, to implantable, total knee replacement prostheses.
The most widely-used type of knee prosthesis for implantation into a patient during a total knee replacement (TKR) procedure includes three components: a metallic, femoral component that attaches to the distal femur; a metallic, tibial component (or tray) that attaches to the proximal tibia; and a polymeric (UHMWPE), insert (also called a bearing or an inlay) that fits between the femoral and tibial components. Various types of patella replacements are also available for use in combination with some of these knee prostheses. Two types of knee prostheses are a posterior-stabilized (PS) prosthesis, for when the posterior cruciate ligament is no longer viable, and a (posterior) cruciate-retaining (CR) knee prosthesis. Each of these two types of knee prostheses may be provided as a fixed bearing knee prosthesis, in which the insert does not move relative to the tibial component, or a mobile bearing knee prosthesis, in which the insert rotates upon a smooth platform of the tibial component. Whether to use a mobile insert or a fixed insert depends largely on the condition of the patient's knee ligaments and other soft tissues.
A knee prosthesis system may include numerous sizes of femoral, tibial and insert components to accommodate the variation of patient anatomies in the worldwide TKR patient population. The design of a knee prosthesis system requires trade-offs among many important factors related to kinematic performance, clinical outcomes, implant longevity, cost, and ease of use, to name just a few. An important consideration relative to both the kinematic performance and the life of the knee prosthesis is the degree of conformity between the femoral component bearing surfaces and the insert bearing surfaces.
Investigators typically characterize conformity in either the coronal plane or sagittal plane as the ratio of the convex radius of a femoral condyle of the femoral component to the concave radius of the interfacing insert surface. A conformity ratio of zero represents a flat insert surface, corresponding to very high contact stress at high loads. A conformity ratio of 0.99 represents high conformity, corresponding, in general, to high contact area, relatively low contact stress and, subsequently, reduced wear rate of the polyethylene surface of the insert.
Investigators have found that conformity in the coronal plane may affect prosthesis life more than conformity in the sagittal plane. For example, in an article by Kuster, et al, “The effects of conformity and load in total knee replacement” (Clinical Orthopaedics and Related Research, Number 375, pp. 302-12, June 2000), the authors found that the compressive surface stress, the shear stress and the von Mises stress were affected by changes to the conformity ratio and to a lesser extent by load changes. In a more recent article by Berend, et al, “Effects of coronal plane conformity on tibial loading in TKA: a comparison of AGC flat versus conforming articulations” (Surgical Technology Int., Number 18, pp. 207-212, 2009), the authors studied the effect of conformity on loading of the proximal tibia of the patient. Improper loading of the proximal tibia may lead to aseptic loosening of the tibial component in the tibia and eventually prosthesis failure requiring revision surgery. The authors found that coronally dished components created a strain increase in the anterior medial tibia while creating a significant strain decrease in the posterior tibia. They also found that proximal tibial strains were decreased and centralized in conforming versus flat articulations.
It is known in the art, however, that very high conformity may also lead, for example, to undesirable loading conditions on the insert surface or to excessive constraint of the femoral component, thereby inhibiting joint motions important to joint performance and patient comfort. Therefore, designs with intermediate values of contact area may be optimal as long as the stresses are below the yield strength of the insert material, in order to provide the optimal combination of joint laxity and conformity.
Complicating the challenge faced by knee prosthesis designers is the variability of patient anatomies in the worldwide, TKR patient population. Smaller patients with smaller femurs require, obviously, smaller knee prostheses. Each of the medial and lateral condyles of a femoral component of a small femoral component has a smaller coronal radius than a large femoral component for a large patient. To maintain the appropriate comformity ratio, as well as other geometrical relationships including condylar spacing, the small femoral component must be matched to a properly sized insert. In addition to the wide range of patient sizes, however, the dimensional proportionality between the femur and tibia bones also varies widely. For example, some patients, have a larger distal femur than other patients for a given size of the proximal tibia. In such cases when using currently available knee prosthesis systems, the surgeon may need to choose to implant a femoral component that is slightly mismatched with the femur and matched with the insert, or a femoral component that is matched with the femur and slightly mismatched with the insert.
Therefore, in view of the foregoing considerations, there is a need for a knee prosthesis system that allows the surgeon to select a femoral component that is sized to fit the femur of a particular patient, a tibial component that is sized to fit the tibia, and an insert that optimally matches the femoral component and is compatible with the tibial component. Such a knee prosthesis system should include both fixed and mobile types of prostheses and provide for both CR and PS procedures. Furthermore, the system should accommodate the wide variety of patient anatomies in the worldwide population.
In addition to providing optimally matched knee prosthesis components, there is an ongoing need to maintain or lower the costs and complexity of knee prosthesis systems. A knee prosthesis system may include femoral, tibial and insert components in a number of sizes, for each of the right and left knees, to accommodate variations in patient anatomies and conditions. In addition, each of inserts may be provided in a number of thicknesses so that the surgeon may select the one that results in the appropriate joint tension. Consequently, knee prosthesis manufacturers must provide a very large inventory of components representing a large number of different size combinations to accommodate the worldwide patient population. What is needed, therefore, is an improved, knee prosthesis system that allows component interchangeability to provide the necessary size combinations with a minimal number of components.
Another consideration during the design of knee prosthesis systems is bone preparation for implantation of the PS femoral component. Both the PS and the CR femoral components have a pair of spaced-apart condyles that are somewhat similar to the natural condyles of the distal femur. For the PS femoral component, a box (or intracondylar notch) positioned between the condyles includes features for interaction with a spine on the PS insert. Implantation of the PS femoral component requires cutting a recess into the distal femur to receive the box. In some current, knee prosthesis systems, the size of the box is the same for all of the PS femoral component sizes, thereby requiring cutting the same size recess into the distal femur, even for smaller femurs. It is desirable, however, to conserve natural bone, if possible, during preparation of the femur for attachment of the femoral component. There is a further need, therefore, for a knee prosthesis system in which each of the PS femoral components has a box that is sized proportionately to the femur size, while also addressing the previously described needs.
Yet another consideration during the design of knee prosthesis systems is bone preparation for implantation of the tibial component. Currently available, mobile and fixed TKR prosthesis systems include tibial components for a range of anatomical sizes. For some of these systems, the tibial component for a mobile TKR prosthesis of a particular size has a different configuration than that of a fixed TKR prosthesis of the same size. Specifically, the platform that supports the fixed bearing insert may have a different shape than the platform that supports the mobile bearing insert. This may result in a small, but possibly significant, difference in coverage of the resected, tibial plateau surface. Although less than ideal, one way surgeons may obtain the desired, tibial bone coverage is to select a larger size tibial component. What is more desirable is a TKR system that has mobile and fixed tibial components with a common platform profile shape that is optimized for interaction with surrounding tissues, kinematic performance, etc.
Also, currently available TKR systems have tibial components with stems of variable lengths to accommodate different tibial bone conditions. Furthermore, the stems for mobile tibial components may have a different configuration than the stems for fixed tibial components. Subsequently, such systems require that a number of different reaming instruments be available for each surgical procedure. A preferable TKR system would have mobile and fixed tibial components with stems of different lengths, but not requiring several different reaming instruments for preparing the tibia. This would also provide the surgeon with the intraoperative flexibility to select the appropriate type of tibial component, while reducing the number of instruments that would need to be available during the surgical procedure.